As processor capabilities have expanded and power requirements have decreased, there has been an ongoing explosion of devices that operate completely independent of wires or power cords. These “untethered” devices range from cell phones and wireless keyboards to building sensors and active Radio Frequency Identification (RFID) tags.
Engineers and designers of these untethered devices continue to have to deal with the limitations of portable power sources, primarily using batteries as the key design parameter. While the performance of processors and portable devices has been doubling every 18-24 months (driven by Moore's law), battery technology in terms of capacity has only been growing at 6% per year.
Even with power conscious designs and the latest in battery technology, many devices do not meet the lifetime cost and maintenance requirements for applications that require a large number of untethered devices, such as logistics and building automation. Today's devices that need two-way communication require scheduled maintenance every three to 18 months to replace or recharge the device's power source (typically a battery). One-way devices that simply broadcast their status without receiving any signals, such as automated utility meter readers, have a better battery life typically requiring replacement within 10 years. For both device types, scheduled power-source maintenance is costly and can be disruptive to the entire system that a device is intended to monitor and/or control. Unscheduled maintenance trips are even more costly and disruptive. On a macro level, the relatively high cost associated with the internal battery also reduces the practical, or economically viable, number of devices that can be deployed.
The ideal solution to the power problem for untethered devices is a device or system that can collect and harness sufficient energy from the environment. The harnessed energy would then either directly power an untethered device or augment a power supply. However, this ideal solution may not always be practical to implement due to low energy in the environment and site restrictions that limit the ability to use a dedicated energy supply.
A need exists for a system that takes these factors into account and provides a solution for the ideal situation and for more restrictive circumstances.
It is known to power a device through inductive coupling (near-field). Power transfer by inductive coupling requires the device to be relatively close to the power transmission source. The RFID Handbook by the author Klaus Finkenzeller defines the inductive coupling region as a distance between the transmitter and receiver of less than 0.16 times lambda, where lambda is the wavelength of the RF wave.
It is known to power a device through harvesting RF waves (far-field). The far-field region is distances greater than 0.16 times lambda.
It is known to power a device using solar, acoustic, vibration, ultraviolet, infrared, thermal, wind, pressure, magnetic, inductive, capacitive, and other types of energy.
A need exists for a system that transfers power using various combinations of types of energy being transmitted and/or received.